I've spent some time thinking about this and Obsidian may be your best bet. The core software is free, and your data is local, so they can't turn around and impose file limits later. You have to pay for their sync, which I don't do - I use FolderSync on Android as my sync solution and it works fine.

Why Obsidian? No software matches all the features of InfoQube, but Obsidian could sync more data elements than other options like LogSeq or Notion can. Although LogSeq and Notion have a block structure, I believe neither of them can do a nested outline and properties for each block simultaneously.

How to set up Obsidian?

1. Each item in IQ could be a page, and the fields can be mapped to Obsidian properties for that page, and the tags could be mapped to Obsidian tags. The contents of the doc pane can be the content of the page, and any embedded pictures or attachments would be picture or attachments in Obsidian.
2. To reproduce a grid with nested items and headings, you could make another page in Obsidian with the name of the grid that contains the content of the grid as nested bullets and headings. It would be ugly in Obsidian though, since each bullet and heading would be a page link. You would also be limited in what you can put in the item field, as the item field would be a page title in Obsidian, so fancy formatting or long text in the item field would not work.
3. Obsidian is becoming like Emacs - an ecosystem in which many developers extend the functionality with plugins. You could make Obsidian do what you need by creating your own IQSync plugin, or IQSync vault structure, and you are not dependent on others to make (and keep) the sync working.
4. Obsidian on mobile has all the plugins enabled, whereas LogSeq does not. If you need to use any Obsidian plugins to make IQ sync work, Obsidian would be better than LogSeq.

I haven't thought about how two-way sync would handle changes made in Obsidian back to IQ, as Obsidian will allow you to add properties on the fly, that would then lack fields in IQ. And if you change the text in an Obsidian "grid" page, how that would translate back if the source grid in IQ has sorting and filtering applied.

Would I use this if it existed? I'm not sure. Since Obsidian is local on my PC next to InfoQube, if I wanted to manage info in Obsidian I would use the native Obsidian program. But then again, I never used the Evernote sync either, so it would be good to get input from people who actually do this and tailor the solution for them, not for me. I'm sharing this as food for thought for this community, since I love using InfoQube and want to inspire others to have good ideas.

Thanks for this update -- this is great news! Would it eventually be possible to have all (or selected) IQ fields for synced items map to YAML properties in Obsidian, with property values editable in Obsidian and 2-way sync?

Meanwhile, here are some interesting approaches to creating databases in Obsidian, which conceivably could work together with IQ sync:

https://archive.ph/Ydgha

You Can Replace Notion With Obsidian. Here’s How… | by Theo Stowell | Obsidian Observer | Jun, 2023 | Medium (archive.ph)

Looking forward to the release of the IQ -> OB sync!

A suggestion: I’m not sure how the sync from IQ fields to Obsidian properties will work. But it wold be helpful if the IQ field ItemParent syncs to an OB property containing  values “[[FIleName]]” instead of “FileName” (that is, each filename in the property  would be surrounded by double brackets). That would allow the plugin Dataview to list the children of an item (although that would require the  user to add a Dataview query manually to each note where one wanted to see a list of children). It would also automatically establish links between parent and child, so that the plugin Backlinks would show a list of children. (But Backlinks would show all the incoming links to the parent, and not just those from its children, and the list wouldn't be quite as nice as that from an appropriate Dataview query.) Either way, this would helpful until such time as there are special grid notes that serve the function of a grid.

Maybe also for ItemGParent.

It's looking good!!!

Minor point: With respect to the ItemParent and Parent, it would help if Parent wasn't an Obsidian property of type "Text", but of type "List" in order to deal with the case in which a note has more than one parent.